1. Field of the Invention
The present invention relates to a control system for a continuously variable automatic transmission, and, more particularly, to a continuously variable automatic transmission control system which controls a gear ratio of a continuously variable automatic transmission according to vehicle driving conditions.
2. Description of Related Art
Typically, continuously variable automatic transmissions are controlled so as to vary its gear ratio by use of gear ratio control maps selected according to vehicle driving conditions. Such a gear ratio control map for the continuously variable automatic transmission defines engine speed control curves for specific engine throttle openings, each of the engine speed control curves specifies a target engine speed according to vehicle speeds. With the gear ratio control maps, the target engine speed is controlled to sharply increase so that the engine generates a great amount of driving torque when the engine throttle opening is relatively large and, on the other hand, is controlled to show a gentle increase so as to improve fuel efficiency when the engine throttle opening is relatively small.
In cases where the transmission gear ratio control is performed by use of the gear ratio control maps, when the engine throttle is fully opened with driver's intention of rapid acceleration, the target gear ratio is set to a maximum gear ratio so that the gear ratio is gradually decreased to increase the speed of vehicle after the engine speed has risen up to the greatest speed in a zone of lower vehicle speeds. This gear ratio control in which an increase in vehicle speed is caused without being accompanied by a change in engine speed makes the driver have a feeling of inept acceleration.
In order for continuously variable automatic transmission of this kind to perform gear ratio control with a result of causing inept acceleration, it has been proposed in, for instance Japanese Unexamined Patent Publication No. 5-332426, to cause a step-wise change in gear ratio by means of repeating alternate gradual increase and rapid decrease in engine speed to increase vehicle speed. This gear ratio control provides a pleasant sensation of driving, such as sporty and light driving sensations, when the engine throttle opening is greater than a specified value. The step-wise gear ratio change possibly occur even during unstable driving, for example driving on a steep slope, turning a corner, driving with greater lateral acceleration, driving with greater longitudinal acceleration, or driving on a road whose surface frictional coefficient is low.